The key elements of a traditional knee-brace or elbow-brace orthosis consist of two pairs of upright bars connected to a pair of hinges centered at the appendage joint; the respective pair of bars extend above and below the appendage joint to be supported; and the respective pairs of the bars are joined to each other with a band going around the half-circumference of the respective lower appendage (below joint) or the upper appendage (above the joint).
The upright bars have traditionally been made from metals, such as steel and aluminum, due to their excellent mechanical strength and modulus, and also due to their ability to be shaped without breaking, which allows the bar to be shaped to fit the patient's appendage. The fitting of a metallic bar to the patient's appendage or to the model cast of the appendage is done by mechanically bending and hammering the metal into shape through trial and error until a desired shape is achieved. Despite care and diligence it is difficult to obtain a completely precise conformance of the bar to the desired shape of the patient's appendage due to the very nature of the bending process, which is long and tedious, relying heavily on the craftsmanship of the technician. When a post-adjustment of the brace is necessary, the process of bending the bar or bars while on the brace is difficult if not impossible.
While the high strength and modulus of the metal provides the bar with excellent performance in being able to withstand high stress and impact loads sustained during usage of a brace, the high specific gravity of the metals makes the brace heavy and therefore uncomfortable to wear. Moreover, many patients requiring a joint orthoses concomitantly have weakened muscles. As such, carrying a heavy brace on the patient's appendage has the effect of contradicting the original intention of the brace--minimizing loads and supporting the weakened appendage and joint.
As an alternative to metal, orthosis brace bars could be fabricated from lightweight composite materials. Fiber reinforced plastic composites have traditionally been made from thermoset resins such as epoxy and polyester. The basic technique involves saturating the fibers or fabric with a liquid resin and then curing or cross-linking the resin to harden it. The cured finished thermoset composite cannot be reheated, softened and shaped.
Attempts have also been made to produce a bar with a partially cured thermoset composite, so that a flat laminate bar could be formed in its soften state, and then shaped. This is a cumbersome process requiring sophisticated, expensive machinery. Moreover, even with this machinery, the results are mixed. Because the bar cannot be resoftened after the final curing, post adjustment of the bar is impossible. For most patients, post-fitting adjustment of the brace is necessary.
While bars made with thermoplastic resin do provide repeated thermoshaping capability, they do not have the required mechanical performance. This is particularly true due to the low flexural strength and low flexural modulus of bars made solely with thermoplastic resin. The desired mechanical performance, however, could be obtained by using high strength and high modulus fibers such as carbon, glass or quartz fibers. Combining these fibers with suitable thermoplastic resin could provide a composite with desired mechanical properties and thermoshaping capabilities. Attempts have been made at making the upright bar using short fibers or discontinuous fibers by pultrusion, injection molding or compression molding. However, discontinuities in the fiber length naturally induce weaknesses in the bar, because the strength of the bar structure becomes critically weakened at the fiber length discontinuities.
Although thermoplastic composites do allow for repeated readjustments, in the application of a composite fiber layer laminate, the heating and shaping results in several problems. The dimensions of the bars generally used for orthosis braces are usually in the range of 16 mm.times.4 mm (width.times.thickness), 18 mm.times.5 mm, and 22 mm.times.6 mm. The ratio of width to thickness typically is 3:1 to 4:1, or alternatively, the bar thickness is approximately 25% to 30% of the width. These dimensions for a composite laminate bar raise several problems relating to the process of heating and shaping the bar.
For example, when the bar is heated, e.g., in an oven, to the melting point of the resin and formed into a shape with a moderate 50 mm radius curvature, several undesirable effects occur to the bar. First, the composite has a tendency to loft, i.e., increase in thickness, due to relaxation of stresses in the compressed laminate. The layers may tend to separate and the overall bar shape generally distorts, that is, it does not remain flat and maintain a rectangular shape.
In order to "reconsolidate," the softened bar into the flat, rectangular shape and recompress the lofted composite layers, uniform pressure needs to be applied to the bar in a controlled way. This is extremely difficult to do. It is found that when a heated bar is shaped with any kind of pressure, the pressure is unevenly imparted to the bar, causing the bar layers to slide and lose the desired rectangular shape. When pressure is applied to recompress the lofted bar, local distortion of the bar often occurs because the pressure is uneven. Accordingly, reconsolidation leads to deformed and unacceptable bars.
Another phenomenon complicating the shaping process is the physical difference in the length of the top surface and bottom surface of a bar shaped into an arc, as shown in FIG. 11(b). The bottom surface naturally is shorter in length than the top surface. However, because the fiber-containing layers cannot alter in length, the bottom surface either wrinkles and buckles, or the fiber-containing layers slide in the longitudinal direction in relation to each other as shown in FIG. 12(a) through FIG. 12(b).
In practice, wrinkling or buckling is unacceptable because such a result evidences a damaged product and is a weakness in the composite. Therefore, the sliding of the layers should be facilitated. However, in the shaping of a composite bar, the uncontrolled sliding of the fiber-containing layers results in disorientation of the fiber-containing layers.
The net result of the above described phenomena is that the shaped bar looses as much as 50% to 80% of its mechanical strength, in addition to having unacceptable aesthetic appearance for use as a brace.
Attempts have been made to control the shaping process by partially melting the resin, so that the shape does not distort easily. While this may result in a "good looking" bar, the strength of the bar is markedly reduced because the sliding of the layers is effectively prevented by some "unmelted resin." Moreover, cracks sometimes result in the bar laminate.
The prior art has also attempted the use of heat shrink tubing to contain the body of the bar. This results in an oval shaped bar because the tubing tends to shrink around the bar with concentric pressure. Moreover, the commonly available heat shrink tubing is soft at the melting temperature of the bar. The pliability of this softened tubing allows uncontrolled distortion of the bar during the shaping process.
Accordingly, there appears to be a need for a lightweight thermoplastic composite laminate bar with high modulus and flexural strength, that is repeatedly thermoshapable for use as the structural components in orthosis braces. There is also a need for a simple method of thermoshaping the composite bar that maintains the high modulus and flexural strength, and the aesthetic appearance of the unshaped bar.